Kagemusha

My take on the significance of a battle - as matteus' - considers also the consequences. And Nagashino, although a battle Katsuyori didn't have to fight at that point of time and space (he had to eventually confront Tokugawa and Oda at some point though, that was pretty inevitable) had major consequences for the history of Japan. So, it cannot be called "pointless". Had Takeda clan won, the gain would be enormous (as was the loss in the defeat).